How should I make scenes with atmospheric lighting brighter?
Whenever you are dealing with volume scatter and volume absorption your light sources will seem not as bright. Depending on the settings used for volume, a lot of the light is lost due to scattering, so it is normal to have lights that are much brighter than normal. 



Without changing your current setup or adding any more lights you can use the ASC-CDL node to change the values for your scene and control the contrast and brightness:



More info on the CDL see this post: What is the the ASC-CDL node?

For even greater and more nuanced control on your image, use the powerful  color management options, you and use custom Lookup Tables (LUTs) to control how the values for the scene are mapped into the sRGB values. 

More info here: Render with a wider dynamic range in cycles to produce photorealistic looking images